1. Field of the Invention
The invention in general relates to marine instruments, and particularly to an oceanographic probe which may be expendable.
2. Description of the Prior Art:
Both military and scientific vessels at sea routinely make measurements of the temperature distribution in the water column beneath the vessel using an instrument called a bathythermograph. The data from these measurements are used to determine the profile of the thermocline, that is, the boundary between the upper mixed water layer and the lower colder water. A knowledge of this thermocline profile adds to the knowledge of ocean circulation, is useful in fishery operations and can be used in the prediction of sonar propagation.
It is often impractical to stop or slow down the vessel to take the measurements due to the tactical situation in a military mission or the economics of ship operation. In order to overcome this limitation a probe has been developed which can be launched over a side of the vessel in motion and which contains a spool of wire which is paid out at a predetermined constant rate. A second spool of wire on board the vessel is paid out as the vessel moves. Such an arrangement is described in U.S. Pat. No. 3,221,556.
Although predictions can be made from the thermocline profile, a much more accurate assessment of sonar propagation, ocean circulation and fishery migration can be obtained with a knowledge of the water salinity.
Basically, salinity is a measure of the salt content of the water, typically seawater, and is a complex function of the temperature, conductivity and pressure of the seawater sample. Salinity can be determined by adding a conductivity cell to the probe and measuring the independent variables of temperature and conductivity and combining them with an indication of pressure, by means of an analog or digital computer. In order to achieve a salinity accuracy of 0.1 parts per thousand it is necessary to measure temperature to 0.05.degree. C and conductivity to 0.05 millimhos, which requires a measuring system accurate to 0.1%. It would be desirable to have an instrument operable in the rigors of an at sea environment for long periods of time which would achieve the desired accuracy for salinity with the use of components having a less stringent accuracy requirement.
In addition, conductivity measurements have inherently nearly instantaneous response whereas temperature measurements inherently have a lag associated with their basic sensing device such as a thermistor. Accordingly the method of utilizing the temperature and conductivity readings to compute salinity cannot be utilized with accuracy in waters having high thermal gradiants.
The present invention allows for an oceanographic probe which provides an indication of salinity to a very high degree of accuracy and which can be used to obtain salinity measurements in the presence of high thermal gradients of the water column.